1.Field of the Invention
This invention relates to an improvement on device which is arranged to control the cumulative storing time of a photo-electric converting element and to process a cumulative signal thereof and is used for the passive type focus detecting device of a camera or the like.
2. Description of the Related Art
An example of the devices of the above stated kind is disclosed in Japanese Laid-Open Patent Application No. SHO 61-167916. In this case, in addition to a CCD sensor consisting of a plurality of picture elements (hereinafter referred to as bits), a monitor sensor is arranged, near to the CCD sensor, to monitor a mean value of brightness of an image, i.e. an average luminance value of an incident light, for the purpose of obtaining an image signal at an adequate level from the CCD sensor. According to the patent application, a signal cumulative storing action must be terminated when an integrated value obtained from the photo current of the monitor sensor reaches a preset level. The application thus has disclosed an arrangement necessary for attaining this purpose. The light receiving area of the above stated monitor sensor, however, is arranged to be in an oblong shape of a length which is, for example, 65% of the line length of the CCD sensor. Therefore, an integrated output obtainable from the same incident light having an unvarying peak luminance value varies according to whether the light is incident on the whole light receiving area of the monitor or only on a part thereof. Despite of the arrangement to terminate the signal storing action without fail at the point of time when the integrated value of the photo current of the monitor sensor reaches the preset value, the output of the monitor sensor fluctuates with the incident light receiving area on the monitor sensor. As a result, the signal cumulative storing action tends to become inaccurate in relation to an incident light luminance pattern. Further, the level of the signal cumulatively stored does not always remain within the dynamic range of an A/D converter which is disposed at the CCD sensor or on the output side thereof. In other words, the signal cumulation might become impossible in precise correlation with the luminance pattern of the incident light if the peak luminance value comes to exceed the dynamic range of the A/D converter. Further, in cases where a signal cumulatively stored at a very low luminance condition of the incident light is used in detecting the focused state of a camera, the accuracy of the result of focus detection is much lower than the result of focus detection made with a signal cumulatively stored under a high luminance condition of the incident light. However, no effective solution of this problem has been employed.